Currently, most FGD processes utilize calcium carbonate prepared with ball mills operated in the "wet" mode. Because grinding takes place by action of falling balls in a limestone slurry, moisture in the coarse material before grinding is not a problem. However, this procedure is not efficient for achieving the particle sizes required for open towers of the type described by Klingspor, Bakke, and Bresowar.
It is known in other contexts that limestone can be efficiently ground to the desired fine particle sizes by grinding dry, such as in roller mills or pulverizers. However, as performed in the past, grinding dry to the desired particle size has required the use of auxiliary heat. This can add significantly to the processing costs. For example, natural gas or other like fuel must be burned in amounts sufficient to supply about 100,000 BTU's per ton of limestone processed. This translates into about 100 standard cubic feet (scf) of natural gas per ton of limestone and requires capital as well as operating costs. In addition to the costs, burning fuel to create the high temperatures necessary for drying can result in the generation of NO.sub.x which is another component of acid rain which must be controlled.
It would be desirable to achieve the benefits offered by dry grinding calcium carbonate for treating SO.sub.x -laden combustion effluents, but eliminate the problem of producing additional air-borne pollutants and decrease the costs associated with the procedure.